Chani B.

asked • 01/06/14

How to you put x^2+y^2=x+2 into the standard form of a circle without graphing?

I know the standard form of a circle is (x-h)^2+(y-k)^2=r^2 but I don't see how to put x^2+y^2=x+2 into that form.

1 Expert Answer

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Tom D. answered • 01/06/14

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Chani B.

Where did the fractions come from? The 1/2 comes from completing the square but what about the 1/4 and the 9/4?
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01/06/14

Tom D.

The 1/4 term is a byproduct of completing the square of X.
 
(X-1/2)^2 = X^2 - X + 1/4
 
In order to preserve equality when completing the square, we need to subtract out the 1/4 term which will bring us back to our original two terms (X^2 - X).
 
It is good practice to expand a completed square to ensure that it is equal to what you started with.  The final form is 
 
(X-1/2)^2 + (Y-0)^2 = (3/2)^2
 
Circle centered at (1/2,0) with radius 3/2
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01/06/14

Tom D.

When we subtract out the 1/4 from the left side, we then combine the constants on the right
 
(X-1/2)^2 -1/4 + Y^2 = 2    (after combining on right)...
(X-1/2)^2 + Y^2 = 2+1/4 = 9/4
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01/06/14

Chani B.

Oh ok!! Thank you!! 
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01/06/14

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